In the industry, the Rochow reaction is typically employed for the synthesis of organohalosilanes such as methylchlorosilanes. That is, reaction of alkyl halides and aryl halides such as methyl chloride and benzil chloride with metallic silicon particles is carried out at 250 to 500.degree. C. in the presence of a copper catalyst and a minor amount of a co-catalyst. Although a variety of by-products usually form in this reaction in addition to the main products, the reaction conditions should be maintained to comply with the supply/demand balance of organochlorosilanes. In particular, this reaction requires to keep a high reaction rate, and a key technology in the synthesis of methylchlorosilanes is to increase the selectivity of the mostly demanded dichlorosilane and a key technology in the synthesis of phenylsilanes is to produce a silane composition matching with the demand.
Industrially, the synthesis of organohalosilanes is generally carried out in a reactor such as a fluidized bed or vibrating fluidized bed while feeding thereto a contact mass consisting of metallic silicon, a copper catalyst and optionally a co-catalyst. This process has many drawbacks including that a long time is taken for activation until the reaction reaches a steady state, the activity decreases with the progress of reaction, the reaction rate and selectivity decreases, the amount of unnecessary by-products such as high-boiling fractions increases, and the reactor and piping are clogged due to agglomeration of the catalyst and co-catalyst, which requires exchange of the contact mass and descaling and cleaning of the reactor. In particular, the conventional Rochow reaction requires a very long time for activation until the reaction reaches a steady state. The steady state, in turn, is relatively short and the yield decreases with the lapse of time. In the synthesis of methylchlorosilanes, for example, there arise problems that high-boiling fractions such as disilanes and undesired products such as trichlorosilane increase due to side-reaction and the exchange of the contact mass in the reactor becomes necessary.
Making investigations on these problems from various viewpoints, we have found that formation of Cu--Si active sites on the surface of metallic silicon follows a long process and hence, takes a long time, and copper continuously deposits on the surface of metallic silicon to form an inactive thick copper layer.
More particularly, in a common process, metallic silicon particles and a copper catalyst are mechanically mixed to prepare a contact mass with which a reactor is charged. After the contact mass is heated under an inert gas stream, methyl chloride is fed thereto to effect activation and reaction. Since the time taken for activation is long, it is naturally necessary to charge the system with a large excess of the copper catalyst and feed the additional contact mass in a high concentration. For this reason, as the reaction proceeds, copper thickly covers the surface of metallic silicon particles, which can reduce the reaction rate and deposition of carbon, and eventually an increase of by-products such as disilanes and a drop of selectivity. Since the active copper catalyst and co-catalyst are used in relatively large amounts, copper and the co-catalyst can be coagulated or the contact mass be bound with copper and the co-catalyst, which prevents effective utilization of copper and the co-catalyst and adversely affects the fluidized bed.
With respect to the contact masses for use in the Rochow reaction for synthesizing organohalosilanes, many improvements in the contact mass including the co-catalyst have been proposed from the composition viewpoint. With respect to the preparation of the contact mass, however, most prior art methods are to simply mix its components. There have been made few proposals for the preparation of a contact mass, based on the action mechanism of the catalyst and/or co-catalyst.
With respect to a contact mass consisting of metallic silicon and a copper catalyst and/or a co-catalyst, Japanese Patent No. 2653700 discloses a method for preparing trimethoxysilane by effecting gas phase reaction using a copper chloride-carrying metallic silicon contact mass. JP-A 9-235114 discloses the preparation of copper silicide-bearing metallic silicon particles. However, the Rochow reaction is reaction between solid phase silicon and gas phase organic halide, and the organohalosilane product volatilizes off from the reaction site as a gas. In the former patent, silicon at the center gradually decreases and the copper concentration and the copper film thickness relatively increase. As a result, the life becomes shortened and the co-catalyst fails to fully exert its effect, and the selectivity of silane is insufficient. In the latter patent, metallic silicon powder and a catalyst are simply mixed to form a contact mass, a reactor is charged therewith, an inert gas is passed through the contact mass to heat it to a reaction temperature (above 250.degree. C.), and an alkyl halide is passed through the contact mass to effect reaction. Basically, this process is substantially the same as the current conventional processes. Even when copper silicide was previously formed as proposed therein, its effect was not found significant.
An object of the invention is to provide a method for preparing a contact mass for the synthesis of organohalosilanes, which contact mass can significantly reduce the time required for activation and even when added in a small amount, sustain reaction activity.